Added support for ignoring specific .less files by prefixing them with an underscore.

- Added support for ignoring specific LESS files by prefixing the file name with an underscore. This is the same mechanism as Visual Studio 2012 uses. Ignoring LESS files can be configured by the setting 'ignorePrefixedFiles'.

- Changed the way settings are read, made a function for doing this. This way you can't forget to read newly introduced settings in the various methods. The result is a dictionary which holds all the setting values.